It is often desirable to control fluid flow in response to an electrical signal. For example, the circulation of engine coolant to an automobile heater can be controlled by opening or closing a passage connecting the engine coolant system to the heater. The pressures in the coolant system may exceed 50 psi, so that any valve must operate at such pressures.
It has previously been proposed to use a poppet valve for closing the coolant passage under solenoid control and opening the passage under spring force. The coolant pressure would help in holding the valve securely closed. The same pressure would resist the opening of the valve under spring action, so it has been further proposed to provide a pilot valve, smaller in size and less affected by coolant pressure, to control a bypass around the main valve seat to reduce the differential pressure across the main valve sufficiently to allow the valve spring to open the main valve. Unfortunately the pilot valve was also subject valve to pressure holding the valve closed when the coolant pressure was in the higher portion of its normal range and therefore was unreliable in operation.